Currently, analogue-digital converters with pipelined architecture are used to convert analogue signals to digital signals. These converters comprise several stages each of which receives an analogue signal and delivers as an output on the one hand a digital word in the direction of a digital processing unit, and on the other hand a residual analogue signal in the direction of the next stage. The digital processing unit receives the digital words from each stage, based on which it generates a final word corresponding to the analogue signal received as an input. When the analogue-digital converter has a resolution of “n bits”, it is said that it converts the values of the input analogue signal to corresponding digital words of n bits.
In general, during the analogue-digital conversion of an input signal, an offset of the output digital signal relative to the input analogue signal appears and can disrupt the performance of the converter. This offset is due to the fact that the various elements of the stages of the converter are not perfect, in other words identical elements contained in different stages do not strictly have the same behavior, mainly because of the differences arising from the manufacture of the elements. These offsets can generate errors in the payload frequency band of the signal.
Specifically, an ideal analogue-digital converter should be able to strictly convert a zero analogue signal to a zero digital word. It turns out that, if a standard converter is short-circuited, and that several output digital values of the converter are measured, the average of the measurements is not zero. Therefore the offset is called the difference between the desired signal and the signal actually delivered by the converter.
Certain techniques make it possible to correct this offset. It is possible to cite for example the retroactive analogue method which consists in digitally measuring the offset of an analogue-digital converter and then in converting this digital measurement, with the aid of a digital-analogue converter of the same resolution to an analogue value. This analogue value that is obtained is then subtracted from the input analogue signal of the analogue-digital converter. But this method requires the use of a digital-analogue converter having at least the same resolution as that of the analogue-digital converter. Moreover, the operation of subtracting the analogue signals is carried out at the input of the analogue-digital converter and generates a noise and a distortion of the analogue signal which disrupts the conversion.
Another technique consists in modifying the resolution of the analogue-digital converter so that the latter generates a digital word having a number of bits greater than or equal to the resolution of the converter. But this technique requires furnishing the converter with additional processing means that are complex and that consume more energy.